(1) Field of the Invention
The present invention relates to a transfer device that performs electrical cleaning as well as to an image forming apparatus using this transfer device.
(2) Description of the Related Art
Conventionally, color image forming apparatuses such as copiers, printers, facsimile machines and the like, include a transfer device for transferring a color toner image formed by the image forming portions, to recording paper. As one of known transfer mechanisms of such a transfer device there is an intermediate transfer system in which toner images of different colors formed at the image forming units corresponding to each color are successively transferred to an intermediate transfer element or a belt so that each toner image is laid over the others (primary transfer) and the thus layered color toner image is transferred onto recording paper by a single transfer step (second transfer).
When a transfer scheme of this kind is adopted, “fogging toner” caused by residual potential other than the toner image transfers to the transfer belt, causing soil on the transfer belt. As a result, the transfer belt needs a cleaning mechanism.
Usually, most of the cleaning mechanisms are constructed of a mechanical structure using a cleaning blade. However, when the transfer belt to be cleaned is thin and soft, it is not beneficial to abut and press a hard blade against the transfer belt of this kind because various problems such as occurrence of wrinkles in the belt, occurrence of meandering of the belt and in the worst case, a rupture of the belt occur. From this viewpoint, there is a disclosure of a technology relating to electrical cleaning for performing preferable cleaning of the toner adhering on the secondary transfer element such as a transfer belt, transfer roller or the like, by alternatively impressing a negative electric field that causes negatively charged toner adhering to the secondary transfer belt to return from the secondary transfer belt to the intermediate transfer belt and a positive electric field that causes positively charged toner adhering to the secondary transfer belt to return from the secondary transfer belt to the intermediate transfer belt (patent document 1: Japanese Patent Application Laid-open 2006-308816).
However, when electric cleaning of the secondary transfer belt (causing the residual toner to return from the secondary transfer belt to the primary transfer belt side) is performed in the above way, this method suffers the problem that if the amount of static charge has lowered due to deterioration of the developer (toner) or exposure of the developer to high humidity environments, the cleaning performance lowers hence the toner fails to return from the secondary transfer belt toward the primary transfer belt.